(1) Field of the Invention
The present invention relates to a servo-control, and to an aircraft having such a servo-control.
(2) Description of Related Art
Conventionally, an aircraft has control members, such as the blades of a rotor providing a rotorcraft with lift or indeed the control surfaces of an airplane, for example.
By using flight controls, a pilot controls the control members of the aircraft. Nevertheless, in order to move the control members it is sometimes necessary to deliver forces that are very large.
Consequently, the linkage connecting a flight control to a control member is sometimes provided with a hydraulic system for providing assistance. The hydraulic system comprises a servo-control enabling the pilot to control the aircraft without difficulty and accurately.
More particularly, a rotorcraft is provided with a main rotor that provides the rotorcraft with at least some of its lift and possibly also with propulsion. In order to control the rotorcraft, a pilot may in particular modify the pitch of the blades of the main rotor.
Consequently, the rotorcraft includes a set of swashplates comprising a non-rotary bottom swashplate and a rotary top swashplate. This set of swashplates is sometimes referred to by the person skilled in the art more simply as the “swashplates”. The non-rotary bottom swashplate is connected to the pilot's flight controls, generally via at least three distinct control linkages, whereas the rotary top swashplate is connected to each of the blades via respective rods.
The pilot sometimes controls the set of swashplates via mechanical control. Nevertheless, the forces the pilot needs to exert in order to move the set of swashplates can be very large, in particular if the weight of the rotorcraft is also large.
Consequently, a servo-control is arranged between an upstream portion and a downstream portion of each control linkage in order to assist a pilot. The pilot then operates the servo-controls without applying significant force via the upstream portion, and then the servo-controls relay the order from the pilot and act on the downstream portion of the linkage.
Likewise a helicopter has a tail rotor in which the pitch of the blades can be adjusted via a servo-control in order to control yaw movements of the aircraft.
In conventional manner, servo-controls comprise a jack having at least an outer cylinder and a power rod. The power rod then includes one control piston per cylinder. Each control piston moves in an internal volume of a cylinder. Thus, each control piston defines a retraction chamber and an extension chamber inside each cylinder.
The term “retraction chamber” designates a chamber that causes the servo-control to retract when that chamber is filled with a fluid. Conversely, the term “extension chamber” designates a chamber causing the servo-control to extend when said chamber is filled with a fluid.
A servo-control is said to be a “movable-cylinder” servo-control when the power rod is held stationary by being anchored to a stationary structure of the aircraft. For example, the power rod may be anchored to a main power transmission gearbox. Under such circumstances, each cylinder slides along the power rod as a function of the pressures that exist in the retraction and extension chambers.
Conversely, a servo-control is said to be a “stationary cylinder” servo-control when each cylinder of the servo-control is held stationary. Under such circumstances, the power rod moves in translation relative to each cylinder as a function of the pressures in the retraction and extension chambers.
Furthermore, each cylinder has a hydraulic directional control valve for feeding the retraction chamber or the extension chamber in the cylinder with fluid depending on the received order.
A piloting order from a pilot is thus transmitted to the hydraulic valve, and it is the hydraulic valve that delivers fluid to the appropriate hydraulic chambers. Depending on the orders given, the hydraulic valve thus delivers hydraulic fluid to each retraction chamber or to each extension chamber, and consequently causes the servo-control to retract or extend.
On an aircraft, servo-controls having at least two cylinders are commonly used for controlling the blades of a main rotor, in particular for safety purposes. Thus, if one cylinder becomes inoperative, as a result of an accidental hydraulic leak, the servo-control remains operational.
Each cylinder is usually dimensioned to be capable of moving a control member even in the event of the other cylinder failing.
Furthermore, it is possible for the blades of the tail rotor to be controlled without hydraulic assistance. Under such circumstances, it is possible to use a single-cylinder servo-control. A failure of the servo-control is then not catastrophic.
Thus, in the event of a failure of a hydraulic system, a movable-cylinder servo-control may behave simply like a link. Each movable cylinder of the servo-control can then be moved by the pilot moving control means, even though the forces that the pilot needs to deliver are greater. In order to assist the pilot, it is then possible to implement a force compensator.
The state of the art includes a servo-control having a single movable cylinder that is provided with a hydraulic directional control valve having an input lever. The input lever is mechanically connected to control means that can be operated by a pilot.
Furthermore, the input lever is hinged directly to a distributor slide that is movable inside a housing of the hydraulic valve. Conversely, the input lever is situated outside that housing. Furthermore, the input lever has only a single degree of freedom to move in rotation relative to the housing.
The distributor slide may be a main slide, with an emergency slide being provided to enable the servo-control to operate in the event of the main slide jamming.
Consequently, a movement of the control means as a result of an order from a pilot causes the input lever to turn, thereby causing the distributor slide to move in translation. The position of the distributor slide within the housing of the hydraulic valve determines the pressures of the hydraulic fluid that exist in the retraction and extension chambers of the servo-control.
Furthermore, the servo-control includes a locking system to enable a servo-control to act as a link in the event of a hydraulic failure.
The locking system includes a movable abutment. This abutment is in the form of a locking finger that is movable between an extended position that it reaches in the absence of hydraulic pressure in the hydraulic valve, and a retracted position that it reaches when the hydraulic pressure exceeds a pressure threshold. A spring urges the locking finger towards its extended position.
The locking finger extends from a root projecting out from the valve housing to a head that slides in an intercommunication system of the hydraulic valve. The intercommunication system puts the retraction chamber and the extension chamber of a cylinder of the servo-control into hydraulic communication when the locking finger is in its extended position.
When the hydraulic pressure in the servo-control is greater than the pressure threshold, the hydraulic fluid present in the intercommunication system exerts a force on the locking finger that is greater than the force exerted by the spring. The locking finger is then in the retracted position and it does not impede the movement of the input lever relative to the hydraulic valve.
Furthermore, the head of the locking finger closes a channel of the intercommunication system in order to separate the retraction chamber hydraulically from the extension chamber of a cylinder of the servo-control.
When the hydraulic pressure becomes less than the pressure threshold, the spring moves the locking finger into the extended position. The root of the locking finger of the movable abutment then impedes movement of the input lever relative to the hydraulic valve by penetrating into a conical reception orifice in the input lever. Furthermore, the head of the locking finger no longer closes the channel of the intercommunication system. Under such circumstances, the retraction chamber and the extension chamber are in hydraulic communication.
Consequently, the position of the input lever is locked in a neutral position relative to the valve housing. A movement of the input lever thus causes the hydraulic valve to move together with the cylinder of the servo-control, in particular by means of the locking finger.
The path followed by forces through the servo-control nevertheless then gives rise to large local excess stresses on the locking finger.
Furthermore, during transitional stages between the extended and the retraced positions of the locking finger, the locking finger rubs with considerable friction against a bearing surface of the orifice for receiving the input lever, because of the particular path followed by the forces via the bearing surface. This friction can rapidly lead to the locking finger becoming damaged. Furthermore, the pilot sometimes senses jolting phenomena during the transitional stages, in particular when the locking finger and the orifice that receives it do not face each other exactly.
Furthermore, since the input lever is situated completely outside the valve housing, the reception orifice runs the risk of being clogged, at least in part, by deposits of unwanted material.
Document GB 728 142 describes a locking system.
That Document GB 728 142 describes a system having a first lever connected to flight controls and second lever connected to a control member. The first lever is hinged to the second lever and also to a hydraulic directional control valve and a movable rod of a servo-control.
Under such circumstances, a locking system includes an angled lever in order to secure the first lever to the second lever.
Document GB 744 476 describes a servo-control having a movable cylinder. Under such circumstances, a piston defines a retraction chamber and an extension chamber that are in fluid flow communication with a hydraulic directional control valve of the servo-control.
That hydraulic valve has a locking system provided with a non-return hydraulic device.
Documents GB 627 737, U.S. Pat. No. 2,597,420, and GB 777 273 are also known.